This invention relates to blades for use in fluid flow machines and, more particularly, to the use of composite materials for the prevention of high frequency stripe mode resonance in the blades of axial flow compressors.
Stripe mode resonance can present a significant problem in the design and development of gas turbine engine fan and compressor blades. Stripe mode resonance is a plate deformation vibratory mode, a high frequency resonance phenomenon associated with the aerodynamic wakes generated by rows of airfoils upstream of the blade row experiencing the stripe mode excitation. It is very local in both stress and deformation, being located primarily in the tip extremity of the blade which undergoes predominantly chordwise bending. In contrast, the low frequency modes of flexure and torsion (flutter) extend over a large portion of the blade and produce significant stresses in the lower portion of the airfoil. In the first flexural mode, the stress is primarily that of spanwise bending while in the first torsional mode, the stress is predominantly torsional shear. Thus, the stripe mode resonance problem is completely different in scope, location and mode shape than the low frequency flutter modes.
Contemporary gas turbine engine compressor and fan blade designs incorporate high stage loading, low aspect ratio airfoils which inherently incur stripe mode resonance which can become a significant problem when the vibratory modes cross airfoil passing frequencies at high speeds and energies. The passing frequencies are the rotational wave frequencies produced by the upstream blade wakes and are the product of airfoil relative velocity times the number of wakes. The cross over, or resonant, condition often produces high cycle fatigue with subsequent airfoil fragmentation, usually across the blade corners at the tip.
Although not as spectacular a vibratory mode or as well publicized as airfoil flutter, the stripe mode resonance has required redesign of many, if not most, modern compressors during their development phases. Due to its high frequency nature, stripe mode resonance is not precisely predictable by current design analysis. Also, many modes occur in the passing frequency spectrums of engine operation and elimination of all such resonances by design is inpractical. Therefore, elimination of those few resonances which development testing discloses to be dangerous is normally provided by design modifications during engine optimization, not by initial design. One modification which has been widely adopted is to cut back the airfoil tip corners to temporarily control a fatigue limiting stripe mode. In other instances, elimination of the stripe mode resonance has required substituting a new inlet guide vane airfoil row having a reduced number of airfoils to eliminate frequency cross-over in the engine operating range. Airfoil thickness increase has also been incorporated in the rotor blades to increase blade frequency so as to move the cross-over resonance to an engine speed above the engine operating range. Such design modifications, although effective, are cost consuming and can potentially delay early engine development if immediate temporary measures are unavailable. Thus, a more effective method of eliminating stripe mode problems is required by the gas turbine engine design community.
It has also been recognized that composite materials offer potential for design improvement in gas turbine engines and their adoption in various engine components has been actively pursued for several years. In particular, numerous attempts have been made to replace the relatively heavy metal blades with blades fabricated essentially entirely of composite materials, particularly in aircraft engine applications where engine weight is a critical design parameter. Composite material technologies have become quite sophisticated and diversified since the early efforts involving glass fibers. Recent efforts have been directed toward the utilization of boron, graphite and other synthetic high strength filaments embedded in lightweight matrices. However, the fact that composite airfoils for gas turbine engines have not been universally employed attests to the difficulty of adapting composite material to this sophisticated technology. Thus, it is also desirable to take advantage of the potential composite materials in the design of improved fan and compressor blades.